Optical scanner devices are well-known in the art and produce machine-readable data signals representative of an object or document being scanned by projecting an image of the document onto a photosensitive detector. The electrical signals produced by the photosensitive detector may then be digitized and processed as necessary to produce an image of the scanned object on a suitable display device, such as, for example, the display of a personal computer. If the object being scanned is text, then the data signals may be converted into text data by a suitable optical character recognition (OCR) program or device.
A typical optical scanner may include illumination and optical systems to accomplish scanning of the object. The illumination system illuminates a portion of the object (referred to herein as a "scan region"), whereas the optical system collects light (referred to herein as "image light") reflected by the illuminated scan region and focuses a small area of the illuminated scan region (referred to herein as a "scan line") onto the surface of the photosensitive detector. By way of example, an optical scanner for scanning written documents may utilize a scan line having a length corresponding to the maximum expected document width, typically about 9 inches. Image data representative of the entire object may be obtained by sweeping the scan line across the entire object, usually by moving the illumination and optical systems with respect to the object, although the reverse is also possible.
By way of example, a typical scanner illumination system may include a light source (e.g., a fluorescent or incandescent lamp or an array of light emitting diodes (LEDs)). A typical scanner optical system may include a slit aperture and lens assembly to focus the image of the illuminated scan line onto the surface of the photosensitive detector. Depending on the particular design, the scanner optical system may also include a plurality of mirrors to "fold" the path of the image light, thereby allowing the optical system to be conveniently mounted within a relatively small enclosure. In order to allow a smaller photosensitive detector array to be used, most optical systems also reduce the size of the image of the scan line that is focused onto the surface of the detector. For example, many optical systems have a lens reduction ratio of about 8:1, which reduces the size of the image of the scan line by a factor of about 8.
The most common type of photosensitive detector device used in optical scanners is the charge coupled device or CCD, although other devices may also be used. A CCD may comprise a large number of light sensitive cells or "pixels," each of which collects or accumulates an electrical charge in response to exposure to light. Since the magnitude of the accumulated electrical charge in any given cell or pixel is related to the intensity and duration of the light exposure, a CCD may be used to detect light and dark spots on an image focused thereon. The charge accumulated in each of the CCD cells or pixels is measured and then discharged at regular intervals known as sampling intervals, which may be about 5 milliseconds or so, although other times may also be used.
The various light sensitive pixels of the CCD detector are typically arranged end-to-end so that they form a linear array of light sensitive pixels. Each pixel in the CCD array thus corresponds to a related pixel portion of the elongate scan line. The individual pixels in the linear array are generally aligned along a "cross" direction, i.e., a direction perpendicular to the direction of movement of the illuminated scan line across the object. The direction of movement of the illuminated scan line across the object is referred to herein as the "scan direction." Each pixel of the linear array thus has a length measured in the cross direction and a width measured in the scan direction. In most CCD arrays the length and width of the pixels are equal, typically being about 8 microns or so in each dimension.
As mentioned above, each pixel in the CCD array corresponds to a related pixel portion of the elongate scan line on the object. The corresponding pixel portion on the elongate scan line is referred to herein as an "native object pixel." A native object pixel has dimensions equal to the dimensions of the corresponding pixel on the linear photosensitive detector array multiplied by the lens reduction ratio of the optical system. For example, in a scanner having a CCD pixel size of 8 microns by 8 microns and a lens reduction ratio of 8:1, the size of the native object pixels will be about 64 microns by 64 microns. The linear array of native object pixels that corresponds to the linear array of CCD pixels is referred to herein as a "native scan line."
While optical scanners of the type described above are widely used, they are not without their disadvantages. For example, the optical systems used in such scanners generally employ several optical elements which may be expensive to manufacture and difficult to align. The lens assembly used to focus the image of the illuminated scan line onto the surface of the photosensitive detector may represent a significant portion of the overall cost of the scanner device. While low cost lens assemblies may be used, the cost savings usually comes at the expense of increased image aberrations, such as astigmatism, coma, etc., which generally decrease overall image quality. Many optical scanners also utilize one or more mirrors to fold the path of the image light. While such mirrors have the advantage of allowing the optical system to be mounted within a relatively small enclosure, they may be difficult to align and may impose strict geometrical relationships between the various components of the scanner.
Another disadvantage associated with the image scanning devices of the type described above is that they are generally only capable of scanning at one native resolution. While this limitation is generally acceptable for most scanning applications, it can be a decided disadvantage if the scanner device is to be used to scan an object that is substantially smaller than the length of the native scan line. For example, as mentioned above, the length of the native scan line in most scanner devices is about 9 inches. However, if it is desired to scan smaller objects, e.g., business cards, slides, small photographs, etc., then the effective resolution of the scanner will be reduced considerably since most of the pixels comprising the native scan line are not available for imaging the smaller object.
One solution to the foregoing problem would be to decrease the size of the native object pixels, thus increase the overall resolution of the scanner. Unfortunately, such high resolution scanners are expensive and require substantial increases in processing time and memory required to process the greatly increased amount of image data. Another solution to the problem would be to provide the scanner with two or more optical systems having different lens reduction ratios which correspond to different scan resolutions. In an alternative arrangement, a single zoom lens may be used to provide different lens reduction ratios, thus different resolutions. While such multiple resolution scanners do exist, the multiple lens and zoom lens optical systems tend to add significantly to the overall cost of the scanner.
Consequently, there remains a need for an optical scanner capable of scanning at two or more different resolutions to accommodate a wider range of object sizes, but without the disadvantages and expenses associated with currently available multiple resolution scanners. Ideally, such a scanner should utilize a relatively simple optical assembly, preferably eliminating the need for multiple lens and mirror systems, which can be complex, difficult to align, and costly.